Occurrence and Interaction ofAspergillus flavuswith Other Fungi on Almonds

Abstract

PHILLIPS, D. J., B. MACKEY, W. R. ELLIS, and T. N. HANSEN. 1979. Occurrence and interaction of Aspergillusflavus with other fungi on almonds. Phytopathology 69: 829-831. The occurrence of the aflatoxin-producing fungi of the Aspergillusflavus spicifera, Fusarium roseum, Rhizopus stolonifer, Cladosporium group (AF) on samples from 89 locations in California was influenced by cladosporioides, Penicillium funiculosum, and Aspergillus ficuum. The other fungi on the kernels. The influence of these other fungi on AF was presence of other fungi with A. flavus or A. parasiticus generally reduced evaluated in 1975 and 1976 in field plots. Fruit of almond cultivars Milo, the number of isolates of AF that were recovered from hulls and kernels. Nonpareil, and Ne Plus were inoculated before hull dehiscence with dry Fungi requiring high moisture appeared antagonistic to AF in colonizing conidia of aflatoxin-producing aspergilli alone or in combination with almond kernels. U. chartarum reduced the isolation of A. flavus most groups of fungi (1975) or individual isolates (1976). Fungi used included significantly. yeasts, Moniliniafructicola, Ulocladium atrum, U. chartarum, Drechslera shelled almonds was obtained from each of 89 orchards in the Many species of fungi may colonize drying fruit of the almond Central Valley of California; 100 surface disinfested and 100 (Prunus dulcis (Mill.) D. A. Webb) (10), including Aspergillus nondisinfested kernels were analyzed for fungi. The relative flavus Link and A. parasiricus Speare (11). These two Aspergillus frequency of occurrence of the various fungi on the kernels was spp. (AF) are widespread on seed and other crops (9,15) and may recorded, and simple correlation coefficients on arcsin square root produce aflatoxins before or after harvest if environmental proportions were computed. conditions are favorable. The AF group has been noted frequently Moisture content was determined on a fresh weight basis for each on almond hulls and kernels, but under natural conditions the fungi sample from the weights of a 50-kernel subsample before and after rarely penetrate the undamaged kernel (5,11,13). In contrast, when oven drying for 48 hr at 86 C. almond fruit was inoculated experimentally in the orchard, the Preparation of inoculum. Fungi isolated from almond hulls were frequency of AF colonization of undamaged kernels was high established in pure culture. Inocula of some fungi other than (2-63%) and depended somewhat on the almond cultivar (11). Aspergillus spp. were prepared from the fungi grown separately on Because of this difference between natural and experimental steam-sterilized carnation straw (10 g of air-dried straw and 15 ml colonization, we evaluated the frequency of occurrence of various of water) in wide-mouth jars closed with muslin. After about 2 wk fungi with AF on almond kernels and initiated experiments on the at room temperature (21 C ± 2 C) the colonized straw, which had interaction among fungi on almond hulls, been air-dried, was crumbled with a glass rod. This crumbled straw was used as inoculum in field-plot testing for antagonism to MATERIALS AND METHODS aflatoxin-producing Aspergillus spp. Conidia and conidial heads of A. flavus or A. parasiticus were Examination of fungi on almonds. Surface disinfested and collected, mixed with dry talc, held several weeks, and used to nondisinfested almond kernels or hulls were tested for the presence inoculate almond fruits. The conidia were from 1 -wk-old toxigenic of fungi. For surface disinfestation, samples were dipped in 70% cultures grown on potato dextrose agar at room temperature. (v/v) ethanol/ H20 for 10 sec, then immediately soaked in 0.5% Inoculation of almond fruits with A. flavus and other fungi. In sodium hypochlorite solution for 5 min. The disinfested samples 1975 and 1976, 45 Milo, Nonpareil, and Ne Plus soft-shelled were aseptically plated on malt-salt medium (MSM) containing almond trees near Fresno, CA, were selected for testing. The trees 7.5% NaCl, 2% malt extract (Bacto, Difco Laboratories, Detroit, were in five blocks, each with three trees per cultivar. About 2 wk MI), and 2% agar. Nondisinfested samples were plated directly on before hull-split, representative fruits were sprayed with 0.01% MSM plus 13 mg/ml of 2,6-dichloro-4-nitroaniline to inhibit sodium hypochlorite solution, rinsed with water, and allowed to